/Engine/Source/Runtime/Slate/Public/Framework/Docking/TabManager.h
Provide a tab that will be the main tab and cannot be closed. Prefer using the version that takes in a TabID so you can set the MainTab before it is actually created
void SetMainTab ( const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< const [SDockTab](API\Runtime\Slate\Widgets\Docking\SDockTab) > & InTab )